Conventionally, in the injection molding machine, molds (stationary mold and movable mold) are secured to platens, such as a stationary platen or a movable platen, and the movable platen is moved and driven so as to approach or isolate to/from the stationary platen, and the molds are closed or opened. While the molds are clamped, a synthetic resin in the molten state is injected into an internal cavity of these molds and a molded article is molded, and then, the molds are opened and the molded article is ejected by an ejector mechanism.
Herein, fixation of the molds to the platens of the injection molding machine is often conducted using a plurality of bolts or a hydraulic clamp device, and recently, a magnetic fixation device to adsorb the molds to the platens due to magnetic force and fix them is practically used.
In the injection molding machine of Patent Literature 1, it is configured such that a clamp plate where a plurality of magnet units (magnetic force generating mechanisms) are incorporated, respectively, is fixed to the stationary platen and the movable platen, respectively, and the molds are fixed to the fixation surface of the clamp plates by the magnetic force generated by the plurality of magnet units. Each of the magnet units has a steel block, a plurality of permanent magnets arranged around the outer circumference of this steel block, an alnico magnet placed on the rear side of the steel block and a coil surrounding the outer circumference of this alnico magnet, and energizing of the coil enables to switch the direction (polarity) of the magnetic field of the alnico magnet.
When the molds are fixed to the fixation surface of the clamp plates, electric power is supplied from the control unit to each magnet unit and the coils are energized for several seconds in the predetermined direction by operating a control panel, respectively, and the alnico magnet and the plurality of permanent magnets form a magnetic circuit where molds become a portion of magnetic path, and the plurality of magnet units are switched to an adsorbing state for adsorbing the molds.
In the meantime, in the case of releasing the fixation of the molds, the coils are energized for several seconds in a direction, which is opposite to that of the case of adsorbing the molds, and the polarity of the alnico magnet is switched to the opposite state, and a magnetic circuit where the molds are not in a portion of the magnetic path is formed and the plurality of magnet units are switched to the non-adsorbing state not for adsorbing the molds.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-169840